1. Field of the Invention
This invention relates to an f.theta. lens used in a light beam scanning apparatus wherein a light beam is deflected by a light deflector and caused to scan a surface. This invention particularly relates to an f.theta. lens which exhibits a small curvature of field and a small chromatic aberration of magnification even at a large deflection angle, which is free of adverse effects from errors in processing and assembling the lens, and with which the light utilization efficiency can be kept high.
2. Description of the Prior Art
In light beam scanning apparatuses wherein light beams are deflected by light deflectors and caused to scan surfaces, f.theta. lenses have heretofore been used as scanning lenses. Specifically, a light beam is reflected and deflected at uniform angular velocity by a light deflector, such as a rotating polygon mirror or a galvanometer mirror. The velocity, at which the light beam scans a surface, should be proportional to the velocity (angular velocity), at which the light deflector deflects the light beam. For this purpose, an f.theta. lens having the characteristics of making the amount of movement of the light beam on the surface, which is to be scanned, proportional to the deflection angle .theta. should be located between the light deflector and the surface, which is to be scanned.
In general, the f.theta. lens is composed of a plurality of lenses. For example, Japanese Unexamined Patent Publication No. 53(1978)-137631 discloses an f.theta. lens which is composed of a single lens having a negative refracting power and a single lens having a positive refracting power. Also, Japanese Unexamined Patent Publication No. 58(1983)-21711 discloses an f.theta. lens which is composed of two lenses having a negative refracting power and a single lens having a positive refracting power.
However, the disclosed f.theta. lens, which is composed of two lenses, has the drawback in that, because the number of lenses is small, the degree of freedom of lens design is low, and it is difficult for an f.theta. lens having a small aberration to be designed. The disclosed f.theta. lens, which is composed of three lenses, has the drawback in that, because the number of lens surfaces is large, much light is reflected by the lens surfaces and the light utilization efficiency becomes low. Also, the three lenses, which constitute the f.theta. lens, are mounted on different lens mounts. Therefore, the performance of the f.theta. lens is readily affected by errors in mounting the three lenses.
In light beam scanning apparatuses, it is desired that the deflection angle be made large. When the deflection angle is large, the light beam scanning apparatus can be kept small in size and the scanning range can be kept large. In such cases, the f.theta. lens should exhibit a small chromatic aberration of magnification even at a large deflection angle. Specifically, in cases where a semiconductor laser (LD) is utilized as the light source, the wavelengths of the laser beam produced by the LD vary due to self-heating of the LD, a change in ambient temperature, or the like. Also, as disclosed in U.S. Pat. No. 4,814,791, in order for the intensity of the laser beam to be modulated over a wide dynamic range, both the natural emission region and the laser radiation region of the LD may be utilized. In such cases, the emission spectrum of the produced laser beam becomes broad. Further, as disclosed in U.S. Pat. No. 4,969,699, in an apparatus wherein laser beams produced by a plurality of LD's are combined and the combined laser beams are used to scan a surface, the wavelengths of the laser beams will differ from one another. In such cases, if the chromatic aberration of magnification is large, the position of the beam spot will deviate from the correct position along the scanning direction due to the fluctuation in the wavelengths of the laser beam. As a result, the image quality of an image, which is recorded with the beam spot on the surface to be scanned or is read out with the beam spot from the surface to be scanned, is affected adversely.
As disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-254110, an f.theta. lens has been proposed which exhibits a small chromatic aberration of magnification. However, the disclosed f.theta. lens is not free of the problems with regard to errors in lens mounting and the light utilization efficiency.